Programmable controllers are electronic computers that are used for the control
of machines and manufacturing processes through the implementation of specific
functions such as logic, sequencing, timing, counting, and arithmetic.
They are also known as programmable
logic controllers (PLCs). Historically,
process control of a single or a few related devices has been implemented
through the use of banks of relays and relay logic for both the control
of actuators and their sequencing. The advent of small, inexpensive microprocessors
and single-chip computers, or microcontroller units, brought process control
from the age of simple relay control to one of electronic digital control
while neither losing traditional design methods such as relay ladder diagrams
nor restricting their programming to that single paradigm. The computational
power of programmable controllers and their integration into networks has
led to capabilities approaching those of distributed control systems, and
plantwide control is now a mixture of distributed control systems and programmable
controllers. Applications for
programmable controllers range from small-scale, local process applications
in which as few as 10 simple feedback control loops are implemented, up
to large-scale, remote supervisory process applications in which 50 or
more process control loops spread across the facility are implemented.
Typical applications include batch process control and materials handling
in the chemical industry, machining and test-stand control and data acquisition
in the manufacturing industry, wood cutting and chip handling in the lumber
industry, filling and packaging in food industries, and furnace and rolling-mill
controls in the metal industry.

Although programmable
controllers have been available since the mid-1970s, developments-such
as the ready availability of local area networks (LANs) in the
industrial environment, standardized hardware interfaces for manufacturer
interchangeability, and computer software to allow specification
of the control process in both traditional (ladder logic) and more
modern notations such as that of finite-state machines-have made
them even more desirable for industrial process control.

Programmable
logic controllers are typically implemented by using commonly available
microprocessors combined with standard and custom interface boards
which provide level conversion, isolation, and signal conditioning
and amplification. Microprocessors used in programmable controllers
are similar or the same as those used in personal computers. The
software of a programmable controller must respond to interrupts
and be a real-time operating system, characteristics which the
typical operating system of a personal computer does not possess.

Perhaps the biggest benefit of programmable
controllers is their small size,
which allows computational power to be placed immediately adjacent
to the machinery to be controlled, as well as their durability,
which allows them to operate in harsh environments. This proximity
of programmable controllers to the equipment that they control
allows them to effect the sensing of the process and control of
the machinery through a reduced number of wires, which reduces
installation and maintenance costs. The proximity of programmable
controllers to processes also improves the quality of the sensor
data since it reduces line lengths, which can introduce noise and
affect sensor calibration.